mimxrt: Add flash storage support with VFS and littlefs filesystem.

This commit adds full support for a filesystem on all boards, with a block
device object mimxrt.Flash() and uos.VfsLfs2 enabled.

Main changes are:
- Refactoring of linker scripts to accomodate reserved area for VFS.  VFS
  will take up most of the available flash. 1M is reserved for code.  9K is
  reserved for flash configuration, interrupts, etc.
- Addition of _boot.py with filesystem init code, called from main.c.
- Definition of the mimxrt module with a Flash class in modmimxrt.[ch].
- Implementation of a flash driver class in mimxrt_flash.c.  All flashing
  related functions are stored in ITCM RAM.
- Addition of the uos module with filesystem functions.
- Implementation of uos.urandom() for the sake of completeness of the uos
  module.

It uses sample code from CircuitPython supplied under MIT license, which
uses the NXP SDK example code.

Done in collaboration with Philipp Ebensberger aka @alphaFred who
contributed the essential part to enable writing to flash while code is
executing, among other things.

ports/mimxrt/main.c

@@ -30,6 +30,7 @@
 #include "py/gc.h"
 #include "py/mperrno.h"
 #include "py/stackctrl.h"
+#include "lib/mp-readline/readline.h"
 #include "lib/utils/gchelper.h"
 #include "lib/utils/pyexec.h"
 #include "ticks.h"
@@ -57,6 +58,24 @@ int main(void) {
         mp_obj_list_append(mp_sys_path, MP_OBJ_NEW_QSTR(MP_QSTR_));
         mp_obj_list_init(MP_OBJ_TO_PTR(mp_sys_argv), 0);
 
+        // Initialise sub-systems.
+        readline_init0();
+
+        // Execute _boot.py to set up the filesystem.
+        pyexec_frozen_module("_boot.py");
+
+        // Execute user scripts.
+        int ret = pyexec_file_if_exists("boot.py");
+        if (ret & PYEXEC_FORCED_EXIT) {
+            goto soft_reset_exit;
+        }
+        if (pyexec_mode_kind == PYEXEC_MODE_FRIENDLY_REPL) {
+            ret = pyexec_file_if_exists("main.py");
+            if (ret & PYEXEC_FORCED_EXIT) {
+                goto soft_reset_exit;
+            }
+        }
+
         for (;;) {
             if (pyexec_mode_kind == PYEXEC_MODE_RAW_REPL) {
                 if (pyexec_raw_repl() != 0) {
@@ -69,6 +88,7 @@ int main(void) {
             }
         }
 
+    soft_reset_exit:
         mp_printf(MP_PYTHON_PRINTER, "MPY: soft reboot\n");
         gc_sweep_all();
         mp_deinit();
@@ -83,19 +103,6 @@ void gc_collect(void) {
     gc_collect_end();
 }
 
-mp_lexer_t *mp_lexer_new_from_file(const char *filename) {
-    mp_raise_OSError(MP_ENOENT);
-}
-
-mp_import_stat_t mp_import_stat(const char *path) {
-    return MP_IMPORT_STAT_NO_EXIST;
-}
-
-mp_obj_t mp_builtin_open(size_t n_args, const mp_obj_t *args, mp_map_t *kwargs) {
-    return mp_const_none;
-}
-MP_DEFINE_CONST_FUN_OBJ_KW(mp_builtin_open_obj, 1, mp_builtin_open);
-
 void nlr_jump_fail(void *val) {
     for (;;) {
     }